With applications ranging from diagnostic procedures to radiation therapy, the importance of high-performance medical imaging is immeasurable. As such, advanced medical imaging technologies are continually being developed. Digital medical imaging technologies represent the future of medical imaging. Digital medical imaging systems produce far more accurate and detailed images of an anatomical object than conventional film-based medical imaging systems, and allow for the further enhancement of an image once an anatomical object is scanned.
Tomography is a two-dimensional radiographic imaging technique in which a cross-sectional image of a selected plane of an anatomical object is obtained, while details in other planes of the anatomical object are blurred. Tomosynthesis is an advanced application in radiographic imaging that allows for the retrospective reconstruction of an arbitrary number of tomographic planes of anatomy from a set of low-dose projection images acquired over a limited angle. The depth information carried by these tomographic planes is unavailable in conventional projection x-ray imaging. In other words, tomosynthesis is an advanced three-dimensional radiographic imaging technique in which several two-dimensional images of an anatomical object are obtained at different angles and/or planes. These two-dimensional images are then reconstructed as a three-dimensional image of the volume of the anatomical object. Unlike conventional projection x-ray imaging techniques, tomosynthesis provides depth information about an area of interest within an anatomical object being imaged, such as a tumor or other anatomical feature. Tomosynthesis enables any number of two-dimensional tomographic image slices to be reconstructed from a single scanning sequence of x-ray exposures, without requiring additional x-ray imaging, thereby making tomosynthesis a desirable characterization tool.
Typically, in digital tomography systems, an x-ray source is positioned on one side of an anatomical object to be imaged, while an x-ray detector (i.e., an amorphous silicon flat panel x-ray detector) is positioned on the opposite side of the anatomical object to be imaged. In amorphous silicon flat panel x-ray detectors, an amorphous silicon array is disposed on a glass substrate and a scintillator is disposed over, and is optically coupled to, the amorphous silicon array. The x-ray source sweeps along a line, arc, circle, ellipse, hypocycloid, or any other suitable geometry, directing a beam of x-ray photons towards the scintillator. The scintillator absorbs the x-ray photons and converts them to visible light. The amorphous silicon array then detects the visible light and converts it into an electrical charge at each pixel. The electrical charge at each pixel of the amorphous silicon array is read out and digitized by low-noise electronics, and is then sent to an image processor. Finally, a two-dimensional cross-sectional image is displayed on a display, and may be stored in a memory for later retrieval. A series of two-dimensional cross-sectional images may be reconstructed using one or more three-dimensional reconstruction algorithms, if desired, to incorporate depth information into a final three-dimensional image.
With respect to digital tomography systems, accurate alignment of the x-ray source with respect to the x-ray detector is critical to adequate image resolution. Phantoms are often used for calibrating and/or validating the alignment of film-based x-ray systems, where it is difficult to quantify x-ray levels or signal levels accurately. However, one drawback associated with film-based x-ray systems is that, typically, they only allow for a visual assessment of image sharpness to be made. Digital radiographic imaging systems, such as digital tomography systems, and any other radiographic imaging systems that allow an image to be digitized for numerical analysis, lend themselves to allowing accurate quantitative measurements of the alignment and/or image resolution or sharpness to be obtained. Accordingly, U.S. patent application Ser. No. 10/755,074, filed on Jan. 9, 2004, and entitled “ALIGNMENT SYSTEMS AND METHODS FOR RADIOGRAPHIC IMAGING SYSTEMS,” which is incorporated in-full by reference herein, provides systems and methods, and simple geometric-shaped phantoms, that utilize discrete spatial and frequency analysis to accurately quantify the mechanical alignment of radiographic imaging systems, thereby allowing for the precise mechanical alignment thereof so that optimal image resolution can be obtained therefrom.
With respect to digital tomosynthesis systems, there are two important image quality characteristics: in-plane resolution and slice thickness. In-plane resolution defines a system's capability to resolve adjacent anatomical objects or anatomical features disposed only a small distance apart in the same plane. As an example, referring to FIG. 1, which illustrates (or closely approximates) a standard line pair phantom 10, well known to those of ordinary skill in the art, higher in-plane resolution means that a system is capable of resolving more line pairs 12. In a clinical context, this means that subtle structures, such as capillaries, microcalcifications, or the like, are capable of being resolved.
Slice thickness, on the other hand, defines a system's resolving power between different planes. Conventionally, radiographic images reflect two-dimensional projections of three-dimensional anatomical objects and, thus, it is difficult to understand the spatial relationship between anatomical features. Because the image quality signature test (IQST) in current imaging products is designed to measure only in-plane resolution, bad pixels, and other detector-specific metrics, it is not suitable for the measurement of slice thickness. Considering the tomosynthesis case, it is now possible to encode the depth information of overlapping/underlying anatomical features with images. As compared to the tomography case, the definition of slice thickness is not obvious for the tomosynthesis case, because tomosynthesis planes lie perpendicular (or oblique) to the x-ray beams. In the tomography case, the image planes lie parallel (or nearly parallel, in the multislice tomography case) to the x-ray beams. Therefore, while tomography primarily employs direct measurement of slice thickness, both direct and indirect measurement are required for tomosynthesis.
Therefore, what is needed is an indirect method, and an associated apparatus, for measuring in-plane resolution and slice thickness. Ideally, this method, and the associated apparatus, would be based on the measurement of modulation transfer function (MTF). Advantageously, such a method, and an associated apparatus, would combine both in-plane resolution and slice thickness in one measurement, be accurate and reliable, be easily automated, and not require costly phantoms.